7. PROJECT SUMMARY/ABSTRACT Disruption of vascular supply (ischemia) can lead to compromised bone regeneration, marked by slowed healing (delayed union) or complete failure to heal (non-union). Two significant areas of concern are ischemic healing in fractures and bone allotransplantation ? a procedure commonly used to treat critically sized skeletal defects. Complications due to disrupted bone healing in ischemic fractures and allografts result in increased healthcare costs and increased patient morbidity and mortality. Therapies that are directed at restoring vascular supply and perfusion in ischemic skeletal injuries will improve healing and patient outcomes. Thrombospondins (TSP) are extracellular matrix proteins and potent endogenous anti-angiogenic factors that are over-expressed in a variety of non-healing wounds. TSPs interact with the CD47 receptor to inhibit angiogenesis and vasodilation. Disruption of the TSP-CD47 interaction to enhance revascularization and healing has been successfully implemented in non-skeletal tissues, but not yet in bone. The long-term objective of this project is to enhance vascularization and bone healing in ischemic fractures and bone allografts by suppression of CD47 expression. To achieve the project objective, Aim 1 will demonstrate that disruption of TSP-CD47 ligand-receptor interaction enhances allograft healing. Sub Aim 1A will examine allograft revascularization, resorption, and remodeling between CD47-/- and WT mice. It will also provide mechanistic data for further insight into the biology of allograft healing. Sub Aim 1B will examine host-graft interaction where WT allografts will be grafted into CD47-/- mice and conversely, CD47-/- graft will be grafted into WT mice. Aim 2 will demonstrate that disruption of TSP-CD47 ligand-receptor interaction using a CD47 morpholino enhances ischemic fracture healing and bone allotransplantation. Aim 2 will provide a model for therapeutic disruption of the CD47-TSP interaction to enhance healing in bone. To block CD47 receptor expression and inhibit the TSP-CD47 interaction, mice will be administered CD47 morpholino to systemically. Sub Aim 2A and Sub Aim 2B will examine the effect of the CD47 morpholino in ischemic fractures and allotransplantation, respectively. Additionally, use of the CD47 morpholino will provide a transnationally relevant model for therapeutic disruption of the CD47-TSP interaction to promote ischemic bone healing, as this class of agents is in human use. This training fellowship will fulfill all three components of the National Institute of Arthritis and Musculoskeletal and Skin diseases' mission to; research the causes, treatment, and prevention of musculoskeletal diseases; train basic and clinical scientists to carry out this research; and disseminate research in these diseases. Overall, this research will advance the understanding of bone healing biology and help develop more efficacious therapies. This work will have direct applications in the treatment of ischemic bone injuries which affect millions of patients annually, and will contribute to reducing healthcare costs and improving patient outcomes.